The objective of the proposed study is to generate novel macrolide and ketolide structures that may be useful as antibiotics themselves, or as chemical intermediates in the generation of new semi-synthetic antibiotics. Macrolides are effective in the treatment of respirtory infections. The new ketolide derivatives are particularly effective against an emerging public health threat, MLSB resistance (virginiamycin-, tylosin-, streptogramins-, and erythromycin-resistant) Streptococcus. pneumoniae. The new structures will be generated by molecular biotailoring, which is an environmentally low-impact fermentation process, as opposed to chemical synthesis. The project will involve eight biotailoring enzymes, and five macrolide- or ketolide-producing bactieral host strains, in novel combinatorial arrays to create structures that have never before been produced or tested for antibiotic activity. The biotailoring enzymes have been chosen based on analogous structure-activity relationships that show them to impart biological activity to the parent molecule. In addition, a new macrolide biosynthetic gene cluster will be cloned and its gene sequenced obtained in part, to identify new biotailoring enzymes for future work in Phase II.